Muscarinic cholinergic receptors mediate the actions of the neurotransmitter acetylcholine in the central and peripheral nervous systems, gastrointestinal system, heart, endocrine glands, lungs, and other tissues. Muscarinic receptors play a central role in the central nervous system for higher cognitive functions, as well as in the peripheral parasympathetic nervous system. Five distinct muscarinic receptor subtypes have been identified, m1–m5. The m1 subtype is the predominant subtype found in the cerebral cortex and is believed to be involved in the control of cognitive functions; m2 is the predominant subtype found in heart and is believed to be involved in the control of heart rate; m3 is believed to be involved in gastrointestinal and urinary tract stimulation as well as sweating and salivation; m4 is present in brain and may be involved in locomotion; and m5, present in brain, may be involved in certain functions of the central nervous system associated with the dopaminergic system.
Conditions associated with cognitive impairment, such as Alzheimer's disease, are accompanied by loss of acetylcholine in the brain. This is believed to be the result of degeneration of cholinergic neurons in the basal forebrain, which innervate areas of the association cortex, and hippocampus, which is involved in higher processes.
Efforts to increase acetylcholine levels have focused on increasing levels of choline, the precursor for acetylcholine synthesis, and on blocking acetylcholine esterase (AChE), the enzyme that metabolizes acetylcholine. Administration of choline or phosphatidylcholine has not been very successful. AChE inhibitors have shown some therapeutic efficacy, but may cause cholinergic side effects due to peripheral acetylcholine stimulation, including abdominal cramps, nausea, vomiting, diarrhea, anorexia, weight loss, myopathy and depression. Gastrointestinal side effects have been observed in about a third of the patients treated. In addition, some ACHE inhibitors, such as tacrine, have also been found to cause significant hepatotoxicity, with elevated liver transaminases observed in about 30% of patients. The adverse effects of AChE inhibitors have limited their clinical utility.
Known m1 muscarinic agonists such as arecoline have also been found to be weak agonists of m2 as well as m3 subtype and are not very effective in treating cognitive impairment, most likely because of dose-limiting side effects.
There is a need for compounds that increase acetylcholine signaling or effect in the brain. Specifically there is a need for muscarinic agonists that are active at various muscarinic receptor subtypes in the central and peripheral nervous system. Furthermore, there is a need for more highly selective muscarinic agonists, such as m1- or m4-selective agents, both as pharmacological tools and as therapeutic agents.